1. Field of the Invention
This invention relates to particle imaging systems and more particularly to methods and apparatus for alignment of charged particle beam projection lithography systems.
2. Description of Related Art
A problem with using charged particle projection optics has been the adverse effects of space charge upon beam projection optics. To ameliorate such effects by enhancing charged particle projection optics, hollow beam annular illumination technology is being developed because hollow beam annular illumination technology can suppress space charge effects dramatically. Hollow beam annular illumination with its reduced space charge problems, permits high throughput charged particle lithography equipment to be produced, because with a hollow beam even a projection system which employs a high beam current can produce well focussed images.
U.S. Pat. No. 5,821,542 of Golladay for xe2x80x9cParticle Beam Imaging System Having Hollow Beam Illuminationxe2x80x9d states that xe2x80x9cthroughput is significantly lower for e-beam exposure systems than for photoexposure systems, thus making e-beam tools too costly for general production.xe2x80x9d The Golladay patent states further xe2x80x9cHigher throughput in e-beam lithography systems can presently be achieved by increasing the e-beam current, but only with an unacceptable degradation in resolution. The degradation in resolution can be attributed to interactions between electrons within the electron beam. The natural repulsion between electrons, due to having charges of the same polarity, causes a number of deleterious effects which limit resolution at the workpiece . . . xe2x80x9d To overcome the above stated problem the Golladay patent describes a charged particle beam imaging system in which an annular aperture comprises a central circular area which is substantially non-transmissive to a beam of charged particles and a first ring shaped area which is substantially transmissive to the beam of charged particles surrounded by a second ring-shaped area which is substantially non-transmissive to the beam.
U.S. Pat. No. 5,834,783 of Muraki et al. for xe2x80x9cElectron Beam Exposure Apparatus and Method and Device Manufacturing Methodxe2x80x9d and U.S. Pat. No. 5,973,332 of Muraki et al. for xe2x80x9cElectron Beam Exposure Method, and Device Manufacturing Method Using Samexe2x80x9d describe an E-beam exposure apparatus including a xe2x80x9chollow beam forming stop whose central portion is shielded . . . xe2x80x9d Muraki et al. states xe2x80x9cSince the space charge effect of hollow electron beam (hollow cylindrical beam) is smaller than that of a nonhollow electron beam (e.g. a Gaussian beam), the electron beam can be brought to focus on the wafer to form a source image free from any blur on the wafer . . . the electron density at the peripheral portion becomes higher than that at the central portion.xe2x80x9d
As is discussed below in more detail, even with the above described advantages of hollow beam systems, there are problems with alignment of the charged particle beam with the apertures which define the configuration of the hollow beam.
A method and apparatus for aligning a charged particle beam with an aperture includes providing a hollow beam aperture means adapted for shaping a charged particle beam into a hollow charged particle beam. Then direct the charged particle beam through the aperture. Provide deflection coils for deflecting the charged particle beam relative to the aperture. Vary the current to the alignment deflection coils while measuring the aperture electrical current generated by charged particles reaching the hollow beam aperture as a function of the current to the alignment deflection coils. Then adjust the current in the alignment deflection coils based on the aperture electrical current measured to center the charged particle beam on the hollow beam aperture. Preferably, separate hollow beam and peripheral beam apertures with associated sensing and current are used to center the beam on respective ones of the apertures.